Reflex klystron having a gridded shielding electrode adjacent the reflector



ay M, 198 J. J. HAMILTON 3,383,544

REFLEX KLYSTRON HAVING A GRIDDED SHIELDING ELECTRODE ADJACENT THE REFLECTOR Filed Feb. 26, 1965 3 Sheets-Sheet 1 REFLECTOR ELECTRON REFLECTION PLANE 300v --l IlI\lll I\|l|l-- /;l \6 Hum 4 l 3 8 F/G j -CAVITY +4oov CATHODE PRIOR ART REFLEX KLYSTRON CHANGE IN '1 I' REFLECTOR VOLTAGE REFLECTOR ELECTRODE CHANGE IN ELECTRON REFLECTION PLANE F/G 2 DISTANCE TOP RESONATOR GRID PLANE NEGATIVE O-+POSITIVE D.C. POTENTIAL PROFILE GRID AND SUPPORT L..- CAVITY 1 I I +40ov 2 CATHODE l/VVE/VTOR JEROME JOHN HAM/LTO/V F/@ 5 BY ATTORNEY y 1968 J. J. HAMILTON 3,383,544

REFLEX KLYSTRON HAVING A GRIDDED SHIELDING ELECTRODE ADJACENT THE REFLECTOR Filed Feb 26 1965 3 Sheets$heet f IV VE/V 70/? JEROME JOHN HAM/L TO/V By d A TTUR/VEY y 14, 1963 J J. HAMILTON 83,

REFLEX KLYSTRON HAVING A GRIDDED SHIELDING ELECTRODE ADJACENT THE REFLECTOR Filed Feb. 26, 1965 5 Sheets-Sheet a ELECTRON v GUN ASSEMBLY W -CHANGE IN REFLECTOR VOLTAGE REFLECTOR PLANE CHANGE IN ELECTRON REFLECTION PLANE --ELECTRON REFLECTION 6 $2 PLANE 8x GRID PLANE DISTANCE TOP RESONATOR GRID PLANE NEGATIVE w POSITIVE DC. POTENTIAL PROFILE N VE/V 70/? JEROME JOH/V HAM/LTO/I/ A TTORNE) Unite States Patent Ofiice 3,383,544 Patented May 14, 1968 ABSTRACT OF THE DESCLGSURE A reflex klystron oscillator having a gridded shielding electrode adjacent to the reflector electrode to reduce the sensitivity of phase variations with the changing reflector electrode voltage in the electronic control of the frequency of oscillation without affecting the bunching or tube parameters.

The present invention relates generally to electron discharge devices and more particularly to a frequency stable reflex klystron oscillator having improved reflector electrode modulation sensitivity.

Conventional reflex ldystron oscillators comprise an electron gun assembly, cavity resonator defining an interaction gap and a reflector electrode disposed adjacent the gap. The electron gun assembly emits a beam of electrons which is projected axially through the resonator toward the reflector electrode. Appropriate biasing potentials are applied to related tube structure to result in initial velocity modulation of the electron beam with some electrons being accelerated while other electrons are decelerated to thereby form electron bunches. On returning to the interaction gap after being reflected the electron beam becomes density-modulated. Such modulation then is dependent on the nature of the reflector electrode region and the magnitude of the initial velocity modulation.

When the reflex klystron is operative the electron beam traverses the gap in the cavity resonator in the first transit and the electromagnetic oscillations generate an voltage. In the region of the reflector electrode the beam is turned around for a second traversal of the interaction gap to thereby result in the bunching and generation of an RF. current in the electron beam. If the relative phase of this current and the RF. gap voltage is in the proper range steady oscillations are sustained. Adjustment of the reflector electrode voltages, therefore, results in optimizing of the electrical parameters. Such adjustment is commonly referred to as electronic tuning and provides for a continuous automatic control of the oscillator frequency to tune the receiver of a set to the transmitter. In this manner the appropriate reflector voltage mode is selected to result in the greatest ethciency of the over-all oscillator.

During the second transit the density modulation of the electron beam generates an electronic admittance and it is the phase of this admittance which is adjusted by variation of the reflector voltage to provide for the conditions of equilibrium in the oscillation of the klystron. When the reflector electrode is adjusted for operation at the peak of the mode the electronic admittance is a pure negative conductance. Variation of the reflector voltage in either direction gives rise to a susceptive component and the oscillation frequency changes to maintain the steady-state oscillation condition.

For the reflex l-dystron oscillator it has been shown in the text, Reflex Klystrons, by Jerome J. Hamilton, Chapman and Hall Ltd, 1958, pp. 122423, that the reflector modulation sensitivity is represented by the following general equation:

where N is the reflector voltage mode (i.e., 2%, 3%, etc.) f is the operating frequency Q;, is the loaded Q of the resonator V is the beam voltage V is the absolute value of the reflector voltage For low reflector modulation sensitivity at a specific frequency, resonator voltage and reflector voltage, the klystron should desirably operate in the lowest reflector voltage mode compatible with the other tube design requirements. Similarly, the value of Q should be made as high as possible.

The value of the Q parameter is as follows:

Qt QU Q.

where Q is the unloaded Q of the resonator and Q is the external Q of the resonator. It is therefore evident that the values for Q and Q should be as high as possible which will detract from certain other features characteristic of the ldystron, particularly in the area of electronic tuning. The sensitivity of phase variation with the changing reflector voltage is therefore of primary consideration in controlling the frequency modulation of reflex klystron oscillators without simultaneously affecting any other characteristics.

Accordingly, an object of the present invention is to reduce reflector modulation sensitivity in reflex klystron oscillators.

Another object of the present invention is the provision of a novel structure for adjusting the reflector modulation sensitivity of reflex ldystron oscillators to meet individual application requirements.

Other objects, features and advantages of the present invention will be evident after consideration of the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic representation of a prior art reflex klystron oscillator;

FIG. 2 is DC. potential profile illustrative of the electrical parameters of a conventional reflex klystron oscillator;

FIG. 3 is a schematic representation illustrative of the features of the structure of the present invention;

FIG. 4 is a detailed cross-sectional view of an illustrative embodiment of the invention;

FIG. 5 is an enlarged schematic representation of the pertinent electrode structure of the present invention;

PEG. 6 is a DC. potential profile of the reflector region incorporating the novel structure ofthe present invention;

And FIG. 7 is a schematic representation of an alternative embodiment.-

Referring to FIG. 1 the general behavior and structure of reflex klystrons will be described. The emitter or cathode 2 is provided with a high negative bias from a source 3 while the cavity resonator 4 is positively biased. The cavity resonator is provided with electron permeable structures in the form of grids 7 and 8 disposed Within openings in oppositely disposed walls of the cavity resonator and define therebetween an interaction gap 6. Adjacent to the top resonator grid 7 is a highly negatively biased reflector electrode 19 to thereby return the stream of electrons for the second transit through the interaction gap 6. In the general operation of the reflex klystron the condition of optimum phase is considerably influenced by the transit time of the electrons in the region of the reflector electrode and top resonator grid. It is therefore desirable to provide in such structures for the rapid changing of the electric field from one that is accelerating the electrons to one that is decelerating. Since the effect of the reflector field is highly critical in the determination of the bunching phenomena of the electrons during the second traversal of the cavity resonator gap this invention is directed .to the area intermediate to the reflector electrode and top resonator grid referred to as the zero equipotentia'l line or electron reflection plane. The time required for electrons to return is inversely proportional to their velocity at injection. Hence, the faster electrons penetrate deeper into this region and take a longer time to return than the slower electron-s. To assist in optimizing the bunching parameter numerous prior art configurations of the reflector electrode include a recessed portion 12 in a portion of the over-all diameter somewhat greater than that of the electron beam. This recessed portion has a focusing effect in the provision of radially inward forces on the electrons to tend to counteract any divergences in the beam.

With reference to FIG. 2 there is shown a DC. potential profile illustrative of the electric field distribution in the critical region within the scope of the present invention. The electron reflection plane would be defined by the turnaround point where the maximum number of electrons is influenced for the return transit through the cavity resonator. It will be noted that over the distance of this region indicated by the arrow 14 the field is ideally linearly disposed from the positive top resonator grid to the highly negative reflector electrode. The area indicated by the arrow 16 shows that changes in the reflector electrode potential to maximize the electronic tuning results in changes in the disposition of the electron reflection plane indicated by the arrow 18. An analysis of these curves indicates that as the reflector electrode is biased for the higher negative potential with respect to the cathode the turnaround point is pushed nearer to the top resonator grid. The extreme sensitivity in the control of the bunching parameter with changing reflector voltage is indicated by the closeness of the spacings of the areas 16 and 18.

In the reflex klystron oscillator art it is necessary to reduce the sensitivity of changes in the electron reflection plane with respect to the field of the reflector electrode. The present invention has provided a novel and unique method of accomplishing this by introducing a shielding grid-controlled electrode in close proximity to the reflector electrode to thereby screen or attenuate the effect of this electrode on the critical region. The shielding electrode is desirably located at the turnaround or electron reflection plane and is provided with a suitable bias utilizing existing power supplies for the cathode or reflector electrodes. FIG. 3 schematically illustrates the novel embodiment in prior art klystron structures. The reflector electrode numerically designated as 19 is provided with a straight edge terminal end 20. Adjacent to this end is a support member 21 which is insulated from the electrode 19. Grid member 22 is transversely disposed to the flow of the electron stream at or near the turnaround point between this electrode and the top resonator grid to thereby lower the sensitivity of changes in the reflector voltages on the DC. potential profile characteristics in the adjacent region. In this embodiment the grid 22, together with support structure 21, is shown biased at the cathode potential which effectively provides a higher negative potential with respect to the reflector electrode. Utilizing the disclosed structure has resulted in a substantial reduction in reflector electrode modulation sensitivity with control of this parameter being determined by the spacing between the reflector electrode and the grid as well as the electron permeability, or transparency, of the latter. The dissipation of any heat generated by electron bombardment of the grid is a negligible factor in view of the fact that the electrons at or near the reflection plane are left with very little residual kinetic energy at the time of contact with the grid structure.

An illustrative embodiment of the present invention will be described referring now to FIGS. 4 and 5. The illustrative embodiment shown comprises a body member 30 fabricated in complementary sections 32 and 34 which lend themselves readily to hobbing techniques, particularly at extremely high microwave frequencies in the order of twenty go. where careful control of extremely small tolerances is essential. Body section 32 is provided with a cathode gun assembly 36 insulatedly supported Within a re-entrant passageway defined therein. The cathode gun assembly is of a conventional construction employing a. cap-shaped emissive member 37 and an internal heater 38. One of the heater terminals is tied to the cathode as shown in FIG. 4, and the heater-cathode connection is coupled to external electrical circuitry by means of lead 39. In this embodiment the connections to external circuitry are disposed right angularly to communicate with a coaxial passageway extending throughout the length of the tube and. exiting by means of a trunk line 40 shown disposed adjacent the upper end. An electron beam focusing member 41 is positioned in close proximity to the electron emitting member at the entrance of the drift space 42. Hollow cavity resonator 44 is defined by means of the recessed walls 46 and the re-entrant portion 47.

Reflector electrode assembly 48 is mounted within body portion 3 The reflector electrode 50 is supported by the ceramic spacer 53 to which itis brazed and which is in turn brazed to member 34. The reflector extension rod 51 which permits external electrical connection to it is supported by hollow tubulation 52. Thus, the upper end of the reflector assembly is supported by ceramic member 54 joined to tubulation 52 with member 54 in turn sealed to annular metallic members 55 and 56 to provide a vacuumtight enclosure for the over-all tube.

The respective body members 32 and 34 are sealed together as by brazing and the microwave frequency output is coupled from the cavity resonator 44 through iris member 57 and a quarter wave transformer section 58 to the output window 59 having a central ceramic member 60. The output window assembly is supported by and recessed within flange member 61 sealed to the respective body portions in the final stages of the overall assembly of the tube structure. The end of the body member adjacent to the cathode gun assembly is provided with a hollow exhaust tubulation 62. vacuum sealed to flange members 63 and 64 to further provide for the retention of the evacuated atmosphere desired in such microwave tubes. After the assembly has been completed and electrical tests performed the opposing ends may be encapsulated as at 65 and 66 with a suitable rubber composition to facilitate operation of the tube under conditions of extreme environment.

In accordance with the teachings of the presentinvention and with reference to the enlarged view shown in FIG. 5 a grid member 70 is supported adjacent the substantially flat end of the reflector electrode 50 in axial alignment with the electron beam emitted from the.

cathode gun assembly shown schematically as 36. Resonator grid 71 extends across the open end of the re-entrant portion 47 of the resonant cavity and the top resonator grid 72 is supported by thewall portion 73 defined in body member 34. Grid member 70 is supported by cylindrical member '74 and insulated from the reflector electrode 50 by means of ceramic spacers to permit independent electrical biasing. The shielding electrode is located at the turnaround point or electron reflection plane which may be readily calculated for the particular design parameters of a selected reflex klystron. .Any modifications may be experimentally determined on a prototype model prior to the final production run.

In the illustration embodiment the shielding electrode has been shown as biased at the cathode electrode potential which means in effect that the grid member is biased at a negative potential approximately 300 volts lower than the reflector electrode. Since a voltage supply is already provided for the cathode gun assembly it is a simple mattcr to run a lead from the grid cylindrical member 74 to tie in with the electrical connection feeding from the trunk line to the cathode gun assembly lead 39. In an experimental embodiment with the shielding electrode tied to the cathode biasing potential, the modulation sensitivity was reduced by a factor of five. It is also permissible in the practice of the invention to provide a separate lead for the shielding electrode to bias same at a potential different from that applied to either the reflector electrode or the cathode. The electrostatic field lines for the electron beam are represented by the equipotential lines 76 and indicate the desired displacement of the grid member 70 with respect to the anode 47.

In FIG. 6 the improved performance of the reflex klystron oscillator with regard to reduction of the sensitivity of the phase variation with changing reflector voltages will be observed. Following the identical D.C. profile representation shown in FIG. 2 the improvement provided by the shielding grid-controlled reflector electrode structure resides in the increased reflector voltage range 80 versus a change in the electron reflection plane 82. For the embodiment illustrated in FIG. 4 without the grid structure and having a fixed frequency of approximately 24.0 gc., a calculated value for reflector modulation sensitivity utilizing the equation disclosed herein would be in the order of 1.10 to 1.45 megacycles per volt. Incorporation of the shielding grid-controlled reflector structure has resulted in a reduction of the modulation sensitivity to a value of approximately 250 kilocycles per volt or a reduction factor of five to six with the grid tied to the cathode supply. Similar improvement in modulation performance was observed with connections to the other supplies biasing the shielding electrode at negative or positive potentials with respect to the cathode.

Tube eificiency was also improved when the shielding electrode was biased 2 volts to l0 volts with respect to the cathode potential. This bias was supplied by means of an external supply coupled between the cathode and shielding electrode. An alternative means for improvement in -R.F. efficiency will be realized by the introduction of a thermoelectric element in series with the cathode and shielding electrode to generate a small current as a result of difference in temperature between two junctions of different metals. Referring to FIG. 7, a thermoelectric element such as a thermocouple 84 is connected between the cathode 2 and grid support 21. Such a modification in the basic inventive concept will obviate the necessity of supplying any additional external voltages to the tube.

The described invention encompasses wide application in the field of parametric amplifier pump devices and communication systems utilizing frequency stable fixed frequency, trimmable or tunable reflex klystrons. Additional features such as improved linearity, power output mode shape control and increased tube efliciency may be realized in all klystron oscillators incorporating the invention. Further, the grid controlled reflector electrode structure provides a second electrode for low power modulation of the output signal of the tube.

While the illustrative embodiment shown and described herein is preferred, numerous modifications will become apparent to those skilled in the art which do not depart from the scope of the broadest aspect of the invention as defined in the accompanying claims.

What is claimed is:

1. A microwave oscillator comprising:

means for emitting an electron beam;

a cavity resonator and a reflector electrode disposed along a common axis;

biasing means including a voltage source connected between said emissive means and cavity resonator for maintaining the former at a negative potential with respect to the latter to thereby cause electrons to be velocity modulated in the direction of said reflector electrode;

voltage biasing means coupled to said reflector electrode to maintain said electrode at a negative potential with respect to said resonator whereby the electron beam is reversed to thereby result in density modulation thereof;

a conductive shielding electrode defining a plurality of electron permeable openings disposed in a plane transverse to the electron beam at a point intermediate to the reflector electrode and cavity resonator; and

means biasing said shielding electrode at a separate negative potential with respect to said reflector electrode to reduce the electron density modulation sensitivity with variations in the reflector electrode electric field.

2. A microwave oscillator according to claim 1 wherein a thermoelectric element is connected in series between the emissive means and shielding electrode to bias the latter at a negative potential of between 2 volts to 10 volts with respect to the former.

3. A reflex klystron tube comprising:

an evacuated envelope;

an electron-emissive cathode, cavity resonator and reflector electrode disposed in axial alignment within said envelope;

a support member surrounding the inner end of said reflector electrode adjacent to said cavity resonator, said support member being joined by insulating means to said reflector electrode;

a wire mesh grid member spaced from the inner end of said reflector electrode and extending transversely between the walls of said support member; and

means including a voltage source biasing said reflector electrode at a negative potential with respect to said cavity resonator and said grid member at a separate negative potential with respect to said reflector electrode to reduce the effect of variations in the reflector electrode electric field on the frequency of oscillations generated within the cavity resonator.

4. A reflex klystron tube according to claim 3 wherein said cathode is biased by means including a voltage source at a negative potential with respect to said cavity resonator and a thermocouple is connected in series between the cathode and grid member to thermoelectrically bias the latter negatively at between 2 volts to 10 volts with respect to the cathode.

References Cited UNITED STATES PATENTS 2,489,156 11/1949 Rigrod 331-84 X 2,659,024 11/1953 Bernier et a1 315-5.19 2,790,928 4/1957 Reed 315-5.21

FOREIGN PATENTS 606,803 7/ 1948 Great Britain.

HERMAN KARL SAALBACH, Primaly Examiner.

ELI LIEBERMAN, Examiner.

S. CHATMON, JR., Assistant Examiner. 

